This application claims priority to Korean Patent Application No. 2005-53017, filed on Jun. 20, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to image sensors, and more particularly, to a pixel circuit with low noise for a global shutter operation in a CMOS image sensor.
2. Description of the Related Art
A CMOS image sensor (CIS) in a cellular phone camera, a digital still camera, etc., converts images into electrical signals, and converts the electrical signals into digital signals. A digital image signal output from the CMOS image sensor is color (Red, Green, or Blue) image data. The digital image signal is further processed for driving a display, such as a LCD.
FIG. 1 is a block diagram of a conventional CMOS image sensor 100. Referring to FIG. 1, the image sensor 100 includes an Active Pixel Sensor (APS) array 110, a row driver 120, and an analog to digital converter (ADC) 130. The row driver 120 receives control signals from a row decoder (not shown), and the ADC 130 receives control signals from a column decoder (not shown).
FIG. 2 is an example of a color filter pattern of the APS array 110 of FIG. 1 when the image sensor 100 is a color image sensor. Each pixel in the APS array 110 has a respective color filter that passes through a certain color of light to be sensed by the pixel. FIG. 2 shows a Bayer color filter pattern having two colors, red (R) and green (G), and another two colors, green (G) and blue (B), alternately arranged in respective rows.
In addition, the G color associated with a brightness signal is assigned to all the rows, and the R color and the B color are alternately assigned to the respective rows so that brightness resolution is enhanced. Typical electronic devices such as digital still cameras use a CIS with millions of pixels for increased resolution.
The APS array 110 senses light using photodiodes that convert light into an electrical signal, thus generating image signals. The analog image signals output from the APS array 110 are for three colors, R, G, and B. The ADC 130 receives the analog image signals from the pixel array 110 for conversion into digital signals using a Correlated Double Sampling (CDS) method. The CDS method is known to those of ordinary skill in the art, and therefore a detailed description thereof is omitted.
FIG. 3 is a circuit diagram of an example pixel circuit 300 within the APS array 110 of FIG. 1. Each pixel in the APS array 100 has a respective pixel circuit that is similar to the pixel circuit 300 of FIG. 3 including a respective photodiode PD and respective four transistors. Such a configuration of the pixel circuit 300 of FIG. 3 is well known to one of ordinary skill in the art.
The pixel circuit 300 performs a rolling shutter operation or a global shutter operation depending on control/supply signals RX, TX, SEL, and VDD. In a rolling shutter operation, rows in a frame are sequentially selected one by one such that signals photo-electrically generated by photodiodes of the selected row are transferred to floating diffusion (FD) nodes to be output as image signals.
In a global shutter operation, signals photo-electrically transformed by all photodiodes of all rows in a frame are transferred simultaneously to FD nodes. Subsequently, rows are sequentially selected for outputting image signals from each selected row at a time.
In either of the rolling or global shutter operations, for each pixel in a row selected by a row selection signal SEL, a reset control signal RX is activated. In that case, a supply voltage VDD is coupled to the FD node for being output as a reset signal VRES. In addition, a signal generated by a photodiode PD is transferred to the FD node for being output as an image signal VSIG when a transfer control signal TX is activated.
Generally, in the rolling shutter operation, after the reset signal VRES is output, the image signal VSIG is output. However, in the global shutter operation, after the image signal VSIG is output, the reset signal VRES is output. Referring to FIG. 1, the ADC 130 performs A/D (analog to digital) conversion on the difference between the reset signal VRES and the image signal VSIG for correlated double sampling. The row selection signal SEL, the reset control signal RX, and the transfer control signal TX may be generated by the row driver 120.
FIG. 4 is a cross-sectional view of a silicon (Si) substrate in which the photodiode PD and the transistors of FIG. 3 are formed according to the prior art. When a transfer control signal TX is activated, a corresponding transistor is turned on for connecting the photodiode PD to the FD node.
For the global shutter operation in FIG. 4, respective charges generated by photodiodes of all rows in a frame are transferred simultaneously to the respective FD nodes. Subsequently, corresponding image signals VSIG are output from a selected row at a time. In such a global shutter operation, a reset signal VRES may not be sampled right before the image signal VSIG is sampled for the pixel circuit 300. Rather, a reset signal VRES may be sampled after the image signal VSIG is sampled, which results in increased noise in the global shutter operation.
Also in the circuit structure of FIG. 4, dark current may flow in the surface region of the Si substrate corresponding to the FD node. In the global shutter operation, respective charges from all photodiodes of all rows in a frame are transferred simultaneously to FD nodes. Such charges are held by each FD node in a standby state until the corresponding pixel is selected for signal output. Thus, in rows having long standby times, the amount of charge stored within the FD nodes may vary from dark current.
In the rolling shutter operation, a standby time period between the charge being transferred to the FD node to being read out as a signal is equal for all rows, and the standby time is short. Thus, the influence of dark current is insignificant. However, in the global shutter operation, dark current may result in picture-quality deterioration.